HUGE Magnatiles fan here! We have a couple of the translucent 100-piece sets and they've been played with more than any other toy for over 2 years. They're great for big spread of ages, too. I can't recommend them highly enough!
If you have a Lakeshore Learning near you, sign up for their e-mail list. They send out 20% and BOGO 1/2 off coupons sometimes that are a great deal. If you order from them online you can still save a little, but the shipping is expensive.
Also, if you have other high-end toy stores in your area, sign up on their mailing lists. We have 2 here locally that carry Magnatiles and they both send out 20% off coupons on their catalogs.